Chevrolet CHEVY VAN G10 SERIES Service Manual page 199

TRA N SM ISSIO N CO N TRO LLED SPA R K SYSTEM
C ontrol o f exhaust emissions em itted by vehicles using the
8
small block V engine, is accom plished by elim inating ignition
vacuum advance w hen the vehicle is operating in low forw ard
gears.
V acuum advance is controlled by a solenoid-operated
switch, w hich is energized by grounding a norm ally open
switch at the transm ission. When the solenoid is in the
non-energized position, vacuum to the d istrib uto r advance unit
is denied and the d istributo r is vented to atm osphere through a
filter at the opposite end o f the solenoid — venting the
d istributor advance u nit prevents it from becom ing locked at
an advanced position. When th e solenoid is energized, the
vacuum po rt is uncovered and the plunger is seated at the
opposite end, shutting o ff the clean air vent.
The vacuum advance solenoid is controlled by tw o switches
and a delay relay. The solenoid is energized in the high
forw ard gear (and in reverse on H ydra-m atic) by a transm ission
o perated sw itch. A therm o static coolant tem perature sw itch is
used to provide therm al override below 82° F.
Wider th ro ttle blade openings at idle are required to
com pensate for the retard ed spark condition produced by the
design o f the emission red uction system . T o prevent engine
dieseling at engine shut dow n, an idle stop solenoid is p ro ­
vided. The ignition activated idle stop solenoid eliminates
dieseling tendencies by allowing th e th ro ttle valve to close
b ey ond the norm al idle position when the ignition is turned
off.
System Theory
T he system com ponents are show n in their norm al a t rest
position w ith the engine o ff and cold (Fig. 6T-11). The
tem perature sw itch points are closed, m aking contact w ith the
cold term inal; the delay relay points are open; transm ission
Fig. 6 T -1 1 —Small V 8 TC S System (Engine O ff)
SMALL V -8 ENGINE
GENERAL DESCRIPTION
switch points are open; idle stop solenoid is de-energized and
plunger retracted ; vacuum advance solenoid is de-energized
w ith plunger shutting o ff th e p o rt to vacuum advance unit.
solenoid is energized, extending the plunger to co n tact the
th ro ttle lever. A circuit is com pleted from the ignition switch
through th e vacuum advance solenoid an d to ground through
the tem perature sw itch (Fig. 6T-12). T he solenoid plunger
moves to open the vacuum p o rt and block the fresh air vent.
The vacuum advance solenoid rem ains energized as long as the
tem perature sw itch cold co n tact rem ains closed.
EMISSION C O N TR O L SYSTEM 6T-7
Fig. 6 T -1 2 —Small V 8 TCS System
(Cold Override Energized)
When the ignition sw itch is tu rn ed on, the idle stop
Fig. 6 T -1 3 —Small V 8 TC S System
(L o w Gear Operation)
C H E V Y V A N S H O P M A N U A L